Binary ionic compounds are formed by the reaction between positive and negative ions of two different elements only and where the positive ion has two different forms or charges. Iron can have two different forms or charges, Fe3+ and Fe2+. Thus, FeCl3 and FeCl2 are two binary compounds.
The positive charge is called the cation while the negative charge is called the anion. When naming a binary ionic compound, the cation is named first followed by the anion. The charge of the cation must be mentioned in Roman numerals in parenthesis. Hence, FeCl3 is named as iron (III) chloride and FeCl2 is iron (II) chloride. Copper forms two positive ions, Cu2+ and CU3+. Thus, copper (III) chloride is a binary ionic compound. Such compounds are referred to as Type II binary ionic compounds.
Unlike Type II binary ionic compounds, Type I binary ionic compounds have cations that have one charge only. The molecule of sodium chloride is an example of a binary compound that is classified as a Type I binary ionic compound. The atom of sodium forms Na+ ion while the atom of chloride forms the Cl- ion. The sodium atom does not have a divalent or a trivalent charge. This makes NaCl a Type I binary ionic compound. Calcium chloride and potassium iodide are other examples of Type I binary ionic compounds.